In furnaces of the above type, the furnace gases are usually collecting under a gas shirt which surrounds the Soderberg anode. The gas shirt is connected to the lower part of the casing and the lower part of the gas shirt extends down to about 5-10 cm above the level of the electrolytic bath. When a solid crust of electrolyte has been formed on the top of the bath, sealing between the lower end of the gas shirt and the crust is obtaining by addition of aluminium oxide on the top of the crust. From the gas shirt, the gas is usually forwarded to a burner where the content of CO in the gas is combusted to CO.sub.2 by the supply of air.
In furnaces equipped with Soderberg anodes, the furnace gas will contain volatiles of pitch which also will be combusted in the burner. From the burner, the combustion gas is forwarded to a gas cleaning unit where the gas is subjected to wet cleaning with water or with a solution which contains alkali or alkali earth compounds, or the gas is subjected to dry cleaning such as absorption on aluminium oxide. The purpose of the cleaning process is to remove dust and gaseous fluorine components from the gas in order to be able to let the gas into the atmosphere without harming the environment. The collected fluorine compounds can be cleaned and returned to the electrolytic furnace.
The above described arrangement for gas collection has a number of disadvantages and drawbacks. One of the disadvantages is that when the crust on the outside of the gas shirt from time to time has to be broken down in order to supply aluminium oxide to the bath, the gas shirt will be open to the atmosphere and furnace gases and dust will escape to the furnace building. If point feeding of aluminium oxide through the gas shirt is used, one will easily have a build-up of oxide under the gas shirt which will prevent the flow of gases in the gas channel under the gas shirt, and there will be a risk of increased gas pressure in the channel which may press the furnace gas out under the gass shirt and into the furnace building. Such a blockage of the gas channel can also happen due to splashing of molten electrolyte. In addition to increased gas pressure and leackage of gases to the furnace building, there may be an increased suction in other parts of the gas channel with the result that aluminium oxide fed by point feeders may be sucked out into the gas collection system. It is a time consuming job to clean a blocked gas channel, and in addition, this job is unpleasant due to the fluorine containing gas and heat stress.
Splashing of the electrolyte bath happens particularly often when the so-called anode effect occurs. The anode effect can be observed by an increase in the furnace voltage from about 5 V to 15-60 V. The reason for the occurance of the anode effect is a build-up of an insulating gas layer on the anode. This gas layer has to be removed in order to stop the anode effect. The strong increase in the voltage can, when an anode effect occurs, cause a partly melting of the side crust in the furnace resulting in an increased electrolytic bath level. The increased bath level together with splashing can result in that the electrolytic bath comes into contact with the gas shirt which strongly cases increased wear of the gas shirt resulting in iron contamination of the produced aluminium. In order to maintain a good environment in the furnace building, the gas shirt, therefore, has to be replaced every second or third year.
It has also been experienced that it is difficult to keep the flanges between each of the gas shirt sections sealed. Air can, therefore, be sucked in between the sections of the gas shirt resulting in combustion of the furnace gases under the gas shirt. This combustion causes an increased wear of the anode and thereby an increased consumption of anode paste.
It has further been found that the sealing obtained by supplying aluminium oxide in the area of the lower end of the gas shirt will not be entirely gas tight. In addition, it is necessary to break the crust in order to tap the produced metal and in order to inspect the anode. By conventional Soderberg operation the complete crust will have to be broken into at regular intervals, for example every second to fourth day in order to charge oxide. A new crust, therefore, has to be built up in order to obtain a sealing of the gas shirt. By point feeding of oxide, this regular breakage of the crust will not be necessary. It will, however, be difficult to close openings made in the crust for allowing tapping of metal etc, as the oxide will flow through such openings without forming a new crust which will block the openings. This causes a pollution of the environment and loss of fluorides which otherwise would have been collected in the gas collection system and returned to the furnace.